Systems for protecting motors from current overload conditions comprising one or more current sensors disposed in a current overload transfer function with respect to percentage of phase current and time are well known in the prior art. Examples of such systems are set forth in U.S. Pat. No. 5,179,364 and 5,179,495. In these systems, an all solid state protection circuit utilizes one or more current transformers which are responsive to the motor phase current. These current transformers are connected into a timer circuit which in turn is used to control the current overload conditions and de-energize a control relay. Current transformers have also been used to measure actual phase current of the motor. For example, a current transformer disclosed in U.S. Pat. No. 4,682,101 is used in a system to provide a DC voltage which is proportional to the AC current flowing into a motor. A control system may use a current transducer to protect against current overload conditions as well as an input system which is used to operate a compressor more efficiently under time varying load conditions.
There are effective systems that protect against improper sequence of a three phase current from being applied to electrical equipment such as scroll compressors. Energization of a scroll or screw compressor with an improper sequence of the three phase power supply can result in mechanical failure. One effective system is set forth in U.S. Pat. No. 5,570,258. In this system, an all solid state protection circuit is used to monitor the voltage level of each phase of the power supply and upon detection of an improper phase sequence or loss of a phase the control is used to de-energize a switch. Although this has proven to be an effective means of protecting against improper phase sequence, there is a need to provide additional functions which provide a more efficient and effective protection system, particularly for screw compressors.
U.S. Pat. No. 4,281,358 shows a protection system having over temperature protection including thermal sensors placed in heat transfer relation with the windings of a dynamoelectric machine which includes a minimum off delay timer to prevent rapid cycling and a low voltage cut-out network to protect the machine in the event of the occurrence of low voltage conditions.
While the above described systems for current overload, over temperature improper phase sequence, off delay timer and low voltage cut-out features have proven to be effective, there still is a need to provide functions which will protect a screw compressor form these and other deleterious conditions in a more effective and efficient manner.